Contraction and Excitation of Smooth Muscles

Arsalan Yousuf

BS 4th Semester

• Involuntary non-striated muscles. • Much smaller than skeletal muscles.

• Found in blood vessels, in lymphatic vessels, the urinary bladder, uterus, male and female reproductive tracts, gastrointestinal tracts, respiratory tracts, arrector pili of the skin, ciliary muscle and iris of the eye.

• Same principles of contraction and relaxation apply with different physical arrangements

Smooth muscles can be divided into two major types:

•Multi unit smooth muscle (operates independantly, innervated by single nerve fiber).

– E.g. ciliary and iris muscle of the eye and Arrector pili muscles of the skin.

•Unitary smooth muscle (operates together as a single unit). Also called syncyctial and visceral smooth muscles.

– Found in walls of gut, bile ducts, ureters, uterus and blood vessels.

Smooth Muscle Types

Smooth muscle contraction

Actin and myosin filaments interact with each other the same way like in skeletal muscles.

•Contractile process is activated by Ca2+

•Energy for contraction is yielded from ATP breakdown.•No troponin complex in smooth muscles.•Myosin filaments have what are called “sidepolar” cross-bridges

Smooth Muscles

Smooth Muscle Contraction as compared to Skeletal Muscle Contraction

• Slow cycling of myosin cross bridges along actin filaments. (1/10 to 1/300)• Slow ATPase activity• Cross bridges remain attached for a longer period of time promoting longer time for

contraction• Small energy is required for sustained contraction (1/10to1/300 of skeletal

muscles)• Slow onset of contraction and relaxation.• Greater force for muscle contraction (4-6 kg/cm2)

Smooth musclesSkeletal muscles

The Latch Mechanism

• Prolonged holding of smooth muscle contractions with little use of energy and little excitatory signal.

• Prolonged tonic muscle contractions can remain for hours.

In place of troponin, smooth muscle cells contain a large amount of another regulatory protein called calmodulin.

CONTRACTION1.Action potential causes depolarization of smooth muscle membrane2.Voltage gated calcium channels open3.Increased Ca2+ enters the cytoplasm through sarcoplasmic reticulum.4.Calcium binds with Calmodulin and then forms a complex with enzyme called calmodulin-myosin light chain kinase.5.The enzyme complex breaks up ATP into ADP and transfers the Pi directly to myosin.6.This Pi transfer activates myosin.7.Myosin forms crossbridges with actin (as occurs in skeletal muscle).

Molecular Mechanism of Smooth Muscle Contraction

Figure from Skeletal Muscle contraction lecture

RELAXATION1.Calcium is pumped out of the cell2.Myosin Light Chain Phosphatase (MLCP) is activated.3.Mysoin is dephosphorylated and disassociates from actin causing relaxation

The Latch Mechanism• Prolonged holding of smooth muscle contractions

with little use of energy and little excitatory signal.• Prolonged tonic muscle contractions can remain for

hours.

Calmodulin

• Calcium binding messenger protein expressed in all eukaryotic cells.• CaM mediates many crucial processes such as:

• inflammation, metabolism, apoptosis, smooth muscle contraction, intracellular movement, short term and long term memory and immune response.

• CaM can also make use of the calcium stores in the endoplasmic reticulum and the sarcoplasmic reticulum.